Quantitatively and qualitatively the prior state-of-the-art is illustrated in FIG. 1, where the volume of the piercing chamber is typically of the order of 15 cc and the encapsulated volume typically varies from 0.01 cc to 0.8 cc. The situation is exemplified by the requirements of Method 1018.2 (Internal Water-Vapor Content) of MIL-STD-883B, which calls for measuring water vapor concentrations of 0.5% with an accuracy of .+-.20% inside ceramic and metal microelectronic packages over the aforementioned volume ranges of 0.01-0.8 cc at an encapsulation pressure of about one atmosphere.
The subsequent discussion will illustrate the difficulties associated with the prior art. Consider a microelectronic device containing an encapsulated, atmospheric-pressure volume of 0.01 cc, containing 0.5% (or 5000 ppm) of water by mole-ratio. This corresponds to about 1.3.times.10.sup.15 water molecules. When the device is , pierced and its contents emerge into a pre-evacuated 15 cc piercing chamber, the new gas pressure, determined by the ideal gas law, is on the order of (0.01/15).times.760 torr, or about 0.5 torr. This configuration presents a number of difficulties:
(1) A reasonably compact volume of 15 cc has a typical dimension, "d", so that d=(15).sup. 1/3 =2.5 cm and a typical internal surface area of the order of 6.times.(2.5).sup.2 =38 cm.sup.2. Water molecules are notorious for their propensity to adsorb into almost any surface, and a single monolayer of sorbed water molecules on a perfectly flat surface contains about 5.times.10.sup.14 molecules/cm.sup.2, or 1.8.times.10.sup.16 molecules in the present illustration. Actual situations are worse; due to microscopic irregularities and pores, the effective surface area may be orders of magnitude larger. It is thus apparent that the internal surface of a typical piercing chamber is capable of sorbing all of the water molecules from the smaller encapsulated chamber wherein the gaseous condition is being analyzed. Conversely, through prior exposure to the atmosphere, the system could easily already be contaminated with as many water molecules as the sample might contain. This represents a source of water molecules that may desorb during analysis to provide an increase of the measured moisture content.
Such problems are very common in analytical chemistry, and particularly in mass spectrometry. Traditional solutions involve the use of glass and/or electropolished metal surfaces to reduce the adverse sorption to monolayer amounts. But, as we have seen, even a monolayer of sorbed molecules may be too much when measuring a minor species of the contents of a very small encapsulation.
In the prior art, these effects can be supposedly minimized by heating the chamber and inlet system. Unfortunately, these efforts are often frustrated because the pre-evacuated heated walls of the chamber/inlet system then exhibit unpredictable sorption activities often with complex hysteresis effects due to the past history of the surfaces One solution is to bake out surfaces in a very reproducible way and to integrate the data over the time required for evacuation of the piercing chamber. Another so-called "dynamic" method is to measure the peak readings of all species just after piercing and attempt to adjust for the fact that sticky molecules such as water are measured at only a small fraction of their true ratio in the original encapsulated sample.
(2) This problem is further greatly compounded by tubing or piping now used to carry the low pressure gas to an analyzer. Because the pressure is low, the tubing must be large in order to provide rapid transport to the analyzer, and this adds still more surface area to the system.
(3) The mean free path for gas molecules at 0.5 torr of pressure is of the order of 0.1 mm, which is a characteristic size of the apertures in valves used to restrict flow into the analyzer to levels compatible with the capacities of their usual vacuum systems. When the mean free path and the aperture sizes are comparable, the flow will be in the transition regime between viscous and free molecular. In free molecular flow, the pumping speed for a gas species is dependent on the molecular velocity, which goes as the inverse square root of the molecular weight. Thus, water is transported more rapidly than, say, oxygen, by the ratio of (32/18).sup.1/2 =1.33. When the flow is in the transition regime, the separation is less severe but is still observed. This means that, even if sorption/desorption were not a problem, the initial measurement made by the analyzer is erroneous and it is necessary substantially to evacuate almost the entire piercing chamber, measuring and integrating all the while, to obtain a meaningful measurement of the relative abundances of species in the original encapsulated sample.
(4) A typical semiconductor may contain about 10.sup.-1 atmospheric-cc of gas, whereas, for the case of a mass spectrometer, typical vacuum systems handle only about 10.sup.-4 atmospheric-cc/sec of inlet gas load. When different gaseous species get separated via sorption and differing pumping speeds that correspond to transition and molecular-flow regimes, it is necessary to analyze all of the gas from the original device, and this can require most of an hour. Productivity and throughput considerations do not normally permit this.
(5) To seal devices of varying shapes onto the outside of the piercing chamber, characteristically elastomer O-rings or gaskets are used, which sorb substantial amounts of water and certain other gases. Similarly, sealing of the piercing shaft is normally accomplished by using elastomer O-rings. Teflon, which has poor elastic properties, may be used with some risk of leakage, but even Teflon sorbs moisture in the small quantities representative of micro-encapsulation measurements.
To compensate for all of these effects, extrapolation techniques are commonly used. These usually assume that adsorption predominates at the time of piercing, and that thereafter the rate of desorption follows a pattern such that if the water signal is tracked for the first few minutes, the amount of water to desorb over the next few hours can be estimated. But this assumption leads to significant inaccuracies because the sorption properties of a surface are complicated functions of the history of that surface, in terms of its state of porosity, oxidation, temperature, exposure to atmosphere, other absorbed or adsorbed species, etc. In actual practice, the system is calibrated empirically against encapsulated calibration standards that are themselves suspect.
The alternative to encapsulated calibration standards is a dynamic calibration using bottled gases (sometimes humidified with trace amounts of water). The technique used in the prior art usually involves evacuating the piercing chamber via a valved port, flooding it with calibration gas through another port, and then sampling the system through a third valve. The presence of all these valves and ports provides additional surface area and introduces O-rings that will probably sorb water and other sticky molecules and further complicate the analysis even when glass and/or electropolished stainless steel are being used to reduce sorption in the main piercing chamber.
It is thus seen that the prior art, by using a large-volume piercing chamber, creates two problems: (1) the unpredictability of sorption dynamics of the chambers' large internal surfaces, and (2) situations wherein flow regimes in the chambers and their inlets are ambiguous mixes of viscous, transition, and molecular flow.
It is thus apparent that there exists a need for a system that can pierce an encapsulated device and transport its contents to an analyzer without the flow-dependent, species-dependent and concentration-dependent effects recited above and the attendant heroic data acquisition analyses required in attempting to compensate for all these complications. Specifically, it would be highly desirable to eliminate all elastomer O-rings, as much Teflon as possible, as many valves and ports as possible, and to reduce, by at least an order of magnitude, the volume of the piercing chamber.